Hi, I don't know how to solve you problem, but the first thing I'd do is send the file contents to the serial monitor.
For example, substitute this line:
MySerial.write(myspritefile.read());
with this:
Serial.print(myspritefile.read(), HEX);
(I suppose MySerial is an instante of the software serial class.)
I think if you do this with a small file, you can easily compare the bytes written by the php script with those that appear on the serial monitor.
HTH